1. Field of the Invention
The present invention generally relates to a two-cycle combustion engine suitable for use as a power plant for a compact working machine such as, for example, a bush cutter and, more particularly, to the two-cycle combustion engine of a design effective to minimize the blow-off phenomenon of exhaust gases in which a portion of the air-fuel mixture used as a scavenging gas is discharged in the form of an unburned gas.
2. Description of the Prior Art
An example of the two-cycle combustion engine of the type referred to above has a scavenging path for supplying a scavenging gas into a combustion chamber including a cylinder-side scavenging passage and a crankcase-side scavenging passage. The crankcase-side scavenging passage is made up of i) a gap defined between an upper inner peripheral surface of the crankcase and the outer peripheral surface of the reciprocating piston and ii) a connecting portion defined between an upper end of this gap and a lower end of the cylinder-side scavenging passage. Also, in this two-cycle combustion engine, in order to facilitate supply of an air-fuel mixture during a high speed engine operating condition, an auxiliary scavenge passage for supplying the air-fuel mixture into the cylinder-side scavenge passage is defined at the interface between left and right components of the crankcase and also at the interface between the crankcase and the cylinder block, to thereby communicate between the interior of the crankcase and the scavenging path. See, for example, the Japanese Laid-open Patent Publication No. 2000-179346, particularly FIGS. 5 and 6 and their related description made therein.
With this construction the prior art two-cycle combustion engine aims at avoiding the blow-off of the air-fuel mixture by allowing the air-fuel mixture within the crankcase to flow towards the cylinder-side scavenge passage through the crankcase-side scavenge passage that is defined by the narrow gap and the connecting portion.
However, it has been found that the cylinder-side scavenge passage employed in the above discussed prior art two-cycle combustion engine has an overall length so small that in a high speed operation of the engine, the velocity of flow of the air-fuel mixture entering the cylinder-side scavenge passage through the gap for the introduction of the air-fuel mixture into the scavenging path in the outer periphery of the reciprocating piston tends to increase and, therefore, the blow-off phenomenon is liable to occur in which the air-fuel mixture, particularly the enriched air-fuel mixture containing a large amount of fuel component and nearly in a liquid phase, is abruptly injected into a combustion chamber from the scavenging port and is subsequently discharged from an exhaust port.
Also, with respect to the supply of the air-fuel mixture during the high speed engine operating condition, although the air-fuel mixture can be supplied from the auxiliary scavenge passage into the cylinder-side scavenge passage, the auxiliary scavenge passage tends to provide a large resistance to flow of the air-fuel mixture and will hardly supply a sufficient amount of the air-fuel mixture into the combustion chamber because of the presence of complicated and tortuous passage portions present in such auxiliary scavenge passage.